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The biggest operational obstacle to the applicationof low-pressure membrane filtration is fouling, i.e.,the reduction of flux or the increase in transmembranepressure (TMP) during operation because ofthe accumulation of materials within the membranepores or on the surface of the membrane. Foulingincreases rapidly once a "critical permeate flux" hasbeen exceeded. However, no standard protocols existfor measuring critical flux or TMP as a function ofcoagulation or changing water quality. Such a protocol,which could provide information analogous tothat provided by jar testing in conventional watertreatment, is necessary. This article describes benchscaletechniques that can be used to measure criticalflux and determine the effects of changing waterquality on membrane performance. The bench-scaletest is also used to demonstrate the effectiveness ofcoagulation on the performance of low-pressuremembrane systems.The concept of critical flux describes the maximumpermeate flux that can be applied without rapid foulingof the membrane. Critical flux has been describedin a theoretical fashion, but the testing procedure proposedin this article relies on an operational definitionof critical flux: the highest flux for which there wasonly a small linear increase in TMP with time of filtration.In the experiments presented in this article, fluxesgreater than the critical flux resulted in exponentiallyincreasing TMP with filtered volume and were labeled"super-critical." The measured value for critical fluxdecreased with increasing time of filtration (for eachstep in the critical flux test). An empirical equationwas developed to allow prediction of critical flux forlonger filtration times.The authors propose that similar bench-scale criticalflux measurements should be used by utilities and consultantsto assist in the design of low-pressure membranesystems, to modify operation during periods ofchanging water quality, and to determine the effects ofcoagulation or other treatments on performance oflow-pressure membrane systems. The authors alsopropose that water utilities and consultants consideradopting bench-scale testing procedures for low-pressurefilters, and it is recommended that results are evaluatedusing plots of average TMP versus flux, withdifferent time steps and with extrapolation of results tolonger time steps. This would provide a relatively conservativeestimate of critical flux. Includes 43 references, table, figures.

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Edition: Vol. 97 - No. 7 Published: 07/01/2005 Number of Pages: 10 File Size: 1 file , 320 KB